[iOS/GN] Add a variable to configure the depth of "bundle_pool".

build/toolchain/mac/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # TODO(brettw) Use "gcc_toolchain.gni" like the Linux toolchains. This requires
 # some enhancements since the commands on Mac are slightly different than on
 # Linux.
 
 import("../goma.gni")
 import("//build/config/clang/clang.gni")
@@ skipping 15 matching lines @@
   compiler_prefix = cc_wrapper + " "
 } else {
   compiler_prefix = ""
 }
 
 if (!use_xcode_clang) {
   compiler_prefix =
       compiler_prefix + rebase_path("$clang_base_path/bin/", root_build_dir)
 }
 
-if (current_toolchain == default_toolchain) {
+declare_args() {
   # Reduce the number of tasks using the copy_bundle_data and compile_xcassets
   # tools as they can cause lots of I/O contention when invoking ninja with a
   # large number of parallel jobs (e.g. when using distributed build like goma).
+  bundle_pool_depth = -1
+}
+
+if (current_toolchain == default_toolchain) {
   pool("bundle_pool") {
-    depth = concurrent_links
+    if (bundle_pool_depth == -1) {
+      depth = concurrent_links
+    } else {
+      depth = bundle_pool_depth
+    }
   }
 }
 
 # When implementing tools using Python scripts, a TOOL_VERSION=N env
 # variable is placed in front of the command. The N should be incremented
 # whenever the script is changed, so that the build system rebuilds all
 # edges that utilize the script. Ideally this should be changed to use
 # proper input-dirty checking, but that could be expensive. Instead, use a
 # script to get the tool scripts' modification time to use as the version.
 # This won't cause a re-generation of GN files when the tool script changes
@@ skipping 312 matching lines @@
 
 mac_toolchain("ios_clang_x86") {
   toolchain_cpu = "x86"
   toolchain_os = "ios"
 }
 
 mac_toolchain("ios_clang_x64") {
   toolchain_cpu = "x64"
   toolchain_os = "ios"
 }